In millimeter wave (mmWave) radio channels, a wide and continuous frequency bandwidth can be available, so that the data transmission rate in this frequency band can be substantially increased. Nevertheless, transmissions in the mmWave radio channel may suffer from serious power path losses. The transmission range of the mmWave signal is usually shorter than the transmission range of the microwave signal. As such, the mmWave signal would have a relatively higher spatial reuse factor. Considering the characteristics of the mmWave channel and the shorter wavelength of the mmWave radio band, a large antenna array can be used to enhance the signal intensity and directivity in specific spatial directions with the array's beamforming gain, or to improve the reliability of signal transmission or channel capacity with the array's spatial diversity or multiplexing gains.
However, in an mmWave radio system, the antennas are in close proximity to each other, due to the short wavelength. Thus, signals transmitted or received by the antennas would have strong spatial correlations, thus reducing the spatial diversity. Further, when a digital beamforming architecture or a precoding technique is applied to the large antenna arrays of a mmWave radio system, the beamformer itself may result in problems of high cost and high power consumption. For example, according to previous researches, power consumption of an analog to digital converter (ADC) with a sampling rate of 500 Hz is between 200 mW and 350 mW. Thus, full digital beamforming with the uses of DAC/ADC will dramatically increase the power consumption and cost of the beamforming architecture. Moreover, the channel capacity gain may not increase proportionately with the number of antennas. Therefore, a hybrid beamforming (HBF) method has become increasingly popular.
Regarding the hybrid beamforming method, it refers to an architecture and method where the beamforming processes can be simultaneously performed at the baseband end and the radio frequency end respectively. Two HBF architectures are often used, which are referred to as the partially connected hybrid beamforming architecture shown in FIG. 1a and the fully connected hybrid beamforming architecture shown in FIG. 1b. 
As shown in FIG. 1a, each radio frequency antenna is connected with a corresponding phase shifter. Each radio frequency antenna can only receive or transmit signals from a fixed analog front end (AFE) and radio frequency (RF) chain.
As shown in FIG. 1b, each radio frequency antenna is connected with multiple phase shifters for receiving or transmitting signals from each AFE-and-RF chain. Theoretically, the fully connected hybrid beamforming architecture provides better signal performance than the partially connected hybrid beamforming architecture.
In short, signals of each AFE-RF chain of the fully connected hybrid beamforming architecture ideally have narrower beams and better array gains. In contrast, the partially connected hybrid beamforming architecture has the advantage of a lower cost, while also have the disadvantage of wider beams and lower array gains. Given the fact that the mmWave radio channel has strong spatial correlations, there is no need to enable all AFE-RF chains at all time, and not all AFE-RF chains require the beam with the narrowest beam width and the highest-gain. Thus, the prior art has to be improved.